The saga begins in the high halls of Asgard, but its roots are found in the deceit of Loki, the trickster. Loki had been flying in the form of a hawk when he was captured by the giant Geirröðr. The giant, recognizing the bird was no mere animal, imprisoned him for three months until Loki, desperate for freedom, promised to lure Thor to Geirröðr’s domain without his hammer Mjölnir, his belt of strength, or his iron gloves. Thor, unaware of the trap and always eager for an adventure in the land of the giants, Jotunheim, agreed to accompany Loki on the journey. They traveled across the desolate landscape where the mountains are like teeth and the wind speaks in the tongues of ancient monsters.
As they ventured further from the safety of the Æsir's realm, they stopped at the house of a giantess named Gríðr. She was not like the hostile giants of Geirröðr’s clan; she was a consort of Odin and the mother of the silent god Víðarr. Gríðr saw the vulnerability of Thor, who walked into the giant’s trap unarmed. She warned him of Geirröðr’s legendary cruelty and the many tricks he would employ. To aid the thunder god, she lent him her own magical artifacts: a pair of iron gauntlets, a belt of power that doubled its wearer's strength, and a legendary wooden staff known as Gríðarvölr. These items were old, imbued with the primordial magic of the world’s creation, and Thor accepted them with a grim determination, realizing that the path ahead was far more dangerous than Loki had led him to believe.
Their journey eventually brought them to the banks of the Vimur, the greatest and most turbulent of the rivers known as the Elivagar. These waters were not merely cold; they were thick with the venom of the world-serpent and the icy sludge of the Niflheim mists. In the region we now recognize as Gullfoss, the river churned with a terrifying violence, its roar echoing off the canyon walls like the scream of a dying beast. Thor, girding himself with Gríðr’s belt and grasping the staff Gríðarvölr firmly, stepped into the water. Loki, terrified of the current, clung to Thor’s belt as the god began to wade through the rising tide.
As they reached the middle of the river, the water did not merely flow; it surged upward with an unnatural speed. It rose from the ankles to the knees, then to the waist, and soon the freezing torrent was lapping at Thor’s shoulders. The thunder god looked upstream, searching for the cause of this sudden, violent deluge. Standing astride the river’s narrowing gorge was Gjálp, the daughter of Geirröðr. She was a giantess of immense proportions, her feet planted firmly on both banks, and it was she who was causing the river to swell. In the ancient telling, she was physically blocking and overflowing the river with her own bodily fluids, a grotesque display of giant-kind's power over the natural elements.
Thor, seeing that his survival depended on stopping the flood at its origin, shouted a phrase that would become a proverb in the North: 'At its source must a river be dammed!' He did not have Mjölnir to throw, but his strength was still that of a god. He reached deep into the riverbed, his fingers clawing through the silt and gravel until he found a massive boulder, a stone the size of a small house. With a heave that caused the very foundations of the earth to tremble, he hurled the rock through the air. The boulder struck Gjálp with such force that she was knocked from her position, her hold over the river broken. The water immediately began to recede, the artificial flood collapsing into a chaotic spray.
However, the danger was not yet over. The receding waters were still fast and treacherous, filled with jagged ice and debris. As Thor struggled toward the far bank, the current nearly swept him off his feet. He felt his grip on Gríðarvölr slipping as the mud gave way beneath him. Just as he reached the edge of the bank, he saw a rowan tree leaning out over the water, its roots clinging tenaciously to the rocky cliffside. Thor reached out and grabbed a low-hanging branch, hauling himself and Loki out of the freezing depths. This moment of salvation was so significant that the Norse people forever after considered the rowan to be a sacred tree, giving rise to the saying, 'Rowan is the salvation of Thor.'
Once on dry land, Thor’s wrath was cold and focused. He proceeded to Geirröðr’s hall, which was a place of iron and shadow. Upon arrival, he was shown to a seat that began to rise toward the ceiling, intended to crush him to death. Using the staff Gríðarvölr, Thor pushed down against the floor with all his might, and a sickening crack echoed through the hall—he had broken the backs of Geirröðr’s daughters, Gjálp and Greip, who had been hiding beneath the chair to lift it. When Geirröðr himself tried to kill Thor by throwing a red-hot iron bolt at him, Thor caught it with the iron gauntlets and hurled it back, the projectile passing through a thick pillar and the giant’s body alike.